1. Field of the Invention
The present invention relates to a data communication method (wireless digital communication method) in which error control is carried out based on an automatic repeat request system.
2. Background of the Invention
As an error control method in data communication, there has been proposed an error control method in an automatic repeat request (ARQ: Automatic Repeat Request or Automatic Request for Repetition) system. According to the error control method based on the automatic repeat request system, if a data frame transmitted from a transmission side to a reception side contains an error, the data frame is again transmitted from the transmission side to the reception side on the basis of the repeat request issued from the reception side to the transmission side.
Various methods have been proposed as the error control method of the automatic repeat request system. Now, a prior art example of an automatic repeat request error control method of a selective repeat (hereinafter referred to as SR) system will hereinafter be described. The selective repeat request system theoretically requires a buffer memory having an unlimited capacity. Thus, such a system is not employed normally, but its improved type is utilized. An automatic repeat request error control method of a primitive SR system will hereinafter be described for making the explanation easy.
Initially, an example of an arrangement of an ARQ frame in which a data frame and a feedback frame have an identical format will be described with reference to FIG. 1. The ARQ frame includes a transmission data frame length A of 16 bits, for example, transmission data B of 584 bits, for example, a transmission data frame number C of 8 bits, for example, and a repeat request frame number D of 8 bits, for example, and the frame is further added with an error detecting code E of 16 bits, for example. This error detecting code E serves for detecting an error which can be contained anywhere in the data.
The transmission data length A can take a value of the data amount of the transmission data B, e.g., values from 0 to 584 in a bit unit. In this case, the data amount thereof is 584 bits. The transmission frame number C indicates a frame number of a frame to be transmitted, and hence the number can be a number from 1 to 255. The repeat request frame number D is utilized in a feedback frame to be transmitted from the reception side to the transmission side, and it indicates a frame number of a frame that is expected to be received next on the reception side, i.e., the earliest frame number that has not been received yet.
The error detecting code E is arranged as a CRC (Cyclic Redundancy check) based on an advice of an ITU (International Telecommunication(s) Union)--T (16 bits). The CRC is effective for detecting an error contained in a region of 624 bits, i.e., the region from the transmission data length A to the repeat request frame number D.
Now, a manner for transmitting an ARQ frame based on the SR system when the transmission frame can be transmitted with failure will be described with reference to FIG. 2 below. As shown in FIG. 2, rectangular frames, each having a numeral written therein, arrayed on the transmission side depict transmission frames, and each numeral written therein designates a frame number. Rectangular frames arrayed on the reception side depict repeat request frames, and each numeral written in each of them also designates a frame number. Although a data frame is originally transmitted from the transmission side to the reception side, there is no data frame originally transmitted from the reception side to the transmission side.
Operations carried out on the transmission side are as follow:
(1) A transmitter on the transmission side transmits transmission frames 1, 2, 3 continuously. PA1 (2) A feedback frame of a repeat request frame number, 2 is received from the reception side. Thus, it is determined that the transmission frame 1 is correctly received on the reception side. A transmission frame 4 is transmitted next. PA1 (3) A feedback frame of a repeat request frame number, 3 is received. Thus, it is determined that the transmission frame 2 is correctly received on the reception side. A transmission frame 5 is transmitted next. PA1 (4) A feedback frame of the repeat request frame number, 3 again is received. Thus, it is determined that the transmission frame 3 does not correctly reach the reception side. Therefore, the transmission frame 3 is again transmitted. PA1 (5) A feedback frame of the repeat request frame number, 3 is again received. Thus, it is also determined that the transmission frame 3 does not correctly reach the reception side. However, since the transmission frame 3 has been already retransmitted, a transmission frame 6 is transmitted next. PA1 (6) A feedback frame of the repeat request frame number, 3 is again received. Thus, it is determined that the transmission frame 3 does not correctly reach the reception side. However, since the transmission frame 3 has been already retransmitted, a transmission frame 7 is transmitted next. PA1 (7) A feedback frame of a repeat request frame number, 6 is received. Thus, it is determined that the transmission frames 3, 4, 5 correctly reached the reception side. PA1 (1) When a receiver on the reception side receives the transmission frame 1 correctly, the feedback frame of the repeat request frame number, 2 is transmitted as a repeat request frame. PA1 (2) The transmission frame 3 is received incorrectly. The repeat request frame number, 3 is transmitted with the feedback frame. PA1 (3) The transmission frames 4, 5 are correctly received. Since the transmission frame 3 has not been received, the feedback frame of the repeat request frame number, 3 is transmitted. PA1 (4) The transmission frame 3, which is again transmitted as the retransmitted frame, is correctly received. Since the transmission frame 5 has been correctly received so far, a feedback frame with the repeat request frame number, 6 is transmitted as a frame that is expected to be received next on the reception side. PA1 (1) The transmission frames 1, 2, 3 are transmitted continuously. PA1 (2) The feedback frame of a repeat request frame number, 2 is received from the reception side. Thus, it is determined that the transmission frame 1 correctly reached the reception side, and the transmission frame 4 is transmitted. PA1 (3) The feedback frame of the repeat request frame number, 3 is received. Thus, it is determined that the transmission frame 2 correctly reached the reception side, and the transmission frame 5 is transmitted. PA1 (4) A feedback frame of the repeat request number, 4 is not received. Thus, it is presumed that the transmission frame 3 did not correctly reach the reception side, and hence the transmission frame 3 is again transmitted. PA1 (5) A feedback frame of the repeat request frame number, 5 is received. Thus, it is determined that the transmission frames 3, 4 have correctly reached the reception side. Therefore, the transmission frame 6 is transmitted. PA1 (6) A feedback frame of the repeat request frame number, 6 is received. Thus, it is determined that the transmission frame 5 has correctly reached the reception side, and the transmission frame 7 is transmitted next. PA1 (1) The transmission frames 1, 2, 3, 4 and 5 are correctly received. The feedback frames of repeat request frame numbers, 2, 3, 4, 5 and 6 are respectively transmitted. PA1 (2) The transmission frame 3 is again correctly received. Since the transmission frame 3 has been already received, the received transmission frame 3 received in an overlapping fashion is discarded. The feedback frame of the repeat request frame number, 6 is transmitted.
Operations carried out on the reception side are as follow:
When the receiver receives the transmission frame 2 correctly, the feedback frame of the repeat request frame number, 3 is transmitted as a repeat request frame.
As described above, according to the error control method of the automatic repeat request system based on the SR system, the transmitter on the transmission side is transmits data frames continuously until it receives the feedback frame indicating that a frame reached the reception side incorrectly. If the transmitter receives information indicative that the frame has reached incorrectly, then it transmits the data frame again. In this manner, the communication system is controlled to recover erroneous data transmission from the transmission side to the reception side.
While in the above case described with reference to FIG. 2, it is assumed that the feedback frame can be transmitted from the reception side to the transmission side without failure, the feedback frame naturally can be transmitted with failure in the actual transmission path, i.e., a space through which electric wave propagates. Now, a manner of the ARQ transmission based on the SR system when the feedback frame is incorrectly transmitted will be described with reference to FIG. 3.
Operations carried out on the transmission side are as follow:
Operations carried out on the reception side are as follow:
As described above, according to the SR system of the prior art example, when the feedback frame fails to be transmitted, leading to the presumption that the corresponding transmission frame is incorrectly received on the reception side, the data frame is transmitted in an overlapping fashion from the transmission side to the reception side in spite of the fact that the data frame has been correctly transmitted from the transmission side to the reception side.
In this case, the ARQ frame is arranged as a full duplex communication system as described above. Therefore, the system is requested to permit data transmission not only from the transmission side to the reception side but also from the reception side to the transmission side at any time. Accordingly, the data frame transmitted from the transmission side to the reception side and the feedback frame transmitted from the reception side to the transmission side have a common format. Therefore, even if there is no transmission data to be transmitted from the reception side with the feedback frame, upon transmission from the reception side to the transmission side, extra information (redundant portion of data) such as "transmission data length", "transmission data", "transmission frame number" and so on have to be included in the feedback frame for maintaining the frame arrangement described with reference to FIG. 1. This leads to a drawback that the reception rate at the transmission side is lowered, resulting in lowering of throughput.
In general, a communication protocol is designed to permit the full duplex communication to be executable. However, in actual practice, communication is often accomplished in a half duplex fashion, i.e., data delivery confirmation is effected in a step-by-step fashion, and hence it is not always necessary to transport data by the feedback frame from the reception side at any time. Therefore, it is undesirable to lower the throughput in spite of the fact that the feedback frame is not always requested to transport transmission data.
Further, in a data communication method in which the data frame transmitted from the transmission side to the reception side and the feedback frame transmitted from the reception side to the transmission side have an identical format, when transmission data to be transported by the data frame or the feedback frame is much shorter (LB.sub.x is taken as its data length) than a predetermined data length (LB.sub.0 is taken as the length which derives from the format), reception rate is lowered upon transmission with the predetermined data length LB.sub.0 maintained as it is.